1. Field of the Invention
This invention relates to an inverter-type resistance welding machine which operates from a high frequency power source. The invention also encompasses a control system and method for the welding machine.
2. Description of the Related Art
In a conventional resistance welding machine, a 50 Hz or 60 Hz alternating current (AC) commercial power source is supplied to the welder through a welding transformer. The level of current flow through the welder electrodes, i.e., weld current, is controlled by phase control of a thyristor with associated control circuitry. Phase control is the process of rapid ON-Off switching which connects an AC supply to a load for a controlled fraction of each cycle. Thus, the duty cycle of the supply is controlled. Control is accomplished by governing the phase angle of the AC wave at which the thyristor is triggered. The thyristor then conducts for the remainder of the cycle.
The use of a relatively low frequency power source (e.g., 50 Hz or 60 Hz) requires a rather large welding transformer in order to provide the requisite weld current. A large transformer necessarily results in a welding machine which is correspondingly larger in size and heavier in weight. In order to reduce the size of the transformer and effect a reduction in the size and weight of the welding machine, one solution is to use a power source having a relatively high frequency, e.g., in the range of 400 Hz to 800 Hz.
The higher frequency power source permits the use of a smaller transformer without sacrificing the amount of weld current which can be delivered to the welding electrodes. Though a higher frequency power source eliminates one problem (the need for a large transformer), it leads to problems in other areas. One such problem has to do with the relatively short period of each cycle of AC voltage due to the higher frequency. The short period of each cycle makes it difficult to provide the necessary phase control over the thyristor because there is insufficient time to consider all of the necessary welding parameters and provide the appropriate phase control response. This problem is especially critical where a microprocessor is used to provide the control function. Accordingly, there is a tendency to refrain from using higher frequency power sources for this reason, the result being that larger size welding transformers are necessary.